The only mention of a person named Carneiro in Guns, Germs, and Steel comes in the notes to Chapter 14. There, Diamond refers to a journal article from 1970 in which Robert Carneiro argues that (in Diamond’s words)

States arise through warfare under conditions in which land is ecologically limiting.

This theory is indeed contrary to what Diamond believes. The main argument of this book as a whole is that agriculture is the key to world history. Diamond believes that areas that were first to develop agriculture were the most likely to become powerful. The creation of centralized states is part of this process of gaining power.

Diamond argues that agriculture leads to a need for centralized government. As agriculture is adopted, higher population densities become possible. When this happens, centralized governments are needed to control the larger populations. As larger become more organized, more agriculture is possible. This leads to still larger populations and, thereby, to more centralized governments. As Diamond says on p. 286 in the paperback edition of the book,

Food production, which increases population size, also acts in many ways to make features of complex societies possible…

In this view, it is successful agriculture, and not conflict over scarce agricultural land, that causes centralized governments to arise.